1. Field of the Invention
This invention generally relates to a new method and a device for the measurement of the oxygen partial pressure in the breath of a human or an animal with an electrochemical sensor and physiologic parameters that can be derived from this.
2. Description of the Prior Art
It has long been known that the analysis of breathing gas can provide much information on the condition of a subject""s pulmonary system and other physiologic parameters on a non invasive basis.
A common solution is the timed collection of breathing gas and the concurrent analysis of the collected fractions for volume and constituents. Needless to say, the discontinuity is an inherent drawback since short-interval concentration changes can not be detected with this method. The measurement of breathing gas constituents on a breath-by-breath basis on the other hand gives much more valuable information on various physiologic parameters that otherwise only can be determined with considerable effort or non direct.
Especially the measurement of oxygen in the breathing gas has a high relevance for the determination of various physiological parameters. The measurement of the partial pressure of oxygen in the breathing gas can be performed with different methods, devices and techniques. Merilainen gives a comprehensive survey of the applied technologies in xe2x80x98Sensors for Oxygen Analysisxe2x80x99, Paramagnetic, Electrochemical, Polarographic and Zirconium Oxide Technologies; Biomedical Instrumentation and Technology, 23, 6, 1989.
In the following one should distinguish between side-stream and mainstream measuring techniques. Side-stream techniques principally can be used with all technologies described hereinafter but they are always affected by the disadvantages of the side-stream method. Only spectroscopic and chemiluminescence methods can be performed on a mainstream basis.
In side-stream methods a pump continuously feeds a breath sample through a tubing system into the analyzer unit. Condensing humidity is trapped into a water trap to avoid interferences in the analyzer. In addition to their technical complexity side-stream methods have inherent analytical disadvantages. Caused by the distance that has to be passed through the tubing system and the water trap, the gas concentrations are measured with a time-shift. This time-shift can not be balanced with the necessary accuracy since different gas compositions show different gas viscosities and therefore have different transport times. Another disadvantage of the side-stream method is the mixing of different gas concentrations on the way to the analyzer, so that the measurement gets inaccurate in relation to the actual concentration in the mainstream of the breathing gas.
Methods based on a paramagnetic principle are known (U.S. Pat. No. 4,464,926) and rely on the paramagnetic effect of oxygen. With this different techniques have been described to convert the oxygen partial pressure into an electrical signal. Usually the breathing gas is mixed with a reference gas in a homogeneous magnetic field. By inversion of the magnetic field the pressure difference between the breathing gas and the reference gas can be measured. So far mainstream measurements are impossible due to the size and the weight of the electromagnet that has to be used. Another disadvantage of paramagnetic sensors is their sensitivity to mechanical shock and other paramagnetic substances like nitrous oxides in the breathing gas. Humidity, that can occur due to temperature changes, can also result in considerable problems. For these reasons paramagnetic methods are only applied in a side-stream mode.
Masspectrometry can also be used for fast oxygen measurements, but the employed instrumentation is very complex. In addition the spectrometer is expensive and needs skilled personnel for its operation. Due to their size masspectrometers also can be used only in a side-stream mode. In practice their use is an exception (xe2x80x98The Medical Mass Spectrometerxe2x80x99; Biomedical Instrumentation and Technology, 23, 6, 1989).
Electrochemical sensors based on solid electrolytes are frequently used for oxygen measurements in industrial applications (e.g. the lambda probe in motor vehicles). For many medical applications they show some inherent disadvantages that limit their use or even make it impractical. To achieve the necessary short response time the sensor has to be heated to at least 500 DEG C (U.S. Pat. No. 4,995,256). Thus for reasons of patient safety their use in the mainstream of the breathing gas is impossible. The heating of the sensor also affords a thermal isolation to the exterior and consumes additional electrical power. This limits the miniaturization of the measuring instrument. Anaesthetic gases in the breathing gas can get decomposed at the heated sensor element so that toxic or dangerous substances are generated. In addition anaesthetic gases or their decomposition products can poison the catalytic surface and thereby cause wrong measuring results.
In mainstream methods the components of the breathing gas are directly measured in the volume stream of the breath. This is preferably done right on the breathing tubus or close to the mouth of the subject. Consequently falsifications of the gas concentration or absorption in the tubing system can be avoided. On the other hand the high humidity of the breathing gas at this sampling point sets up higher demands for the measuring technique.
The measurement of carbon dioxide in the mainstream of the breathing gas on a breath-by-breath basis is a common practice since several years due to the development of small and fast carbon dioxide sensors. For this reason a comparable measuring technique for oxygen is very desirable.
Spectroscopic methods make use of the absorption of electromagnetic radiation by the different breathing gases (EP 0692222). Oxygen molecules absorb electromagnetic radiation in accordance to the radiation frequency. To generate the necessary intensity lasers (e.g. as laser diodes) are used. Alternative the raman effect of oxygen can be used (xe2x80x98Raman Scattering in the Operating Room: Advantages, Specifications and Future Advancesxe2x80x99; Biomedical Instrumentation and Technology, 23, 6, 1989). Methods that employ laser diodes need a temperature regulation and therefore require sophisticated electronic instrumentation (U.S. Pat No. 5,625,189, U.S. Pat. No. 5,448,071). The application of the raman principle needs a high radiation intensity and therefore also requires complex electronic control units and signal processing. Optical oxygen sensors that operate in the UV range are of minor importance since they show too many cross sensitivities. All spectroscopic methods that can be used for the partial pressure measurement of oxygen are principally sensitive to condensing humidity and other components of the breathing gas. They require sophisticated technical equipment and are used for research especially instead of routine use in clinical practice or at the doctors office. Main disadvantages are high production and maintenance costs as well as the need for skilled personnel for instrument operation. Their high complexity and their large size further limit their use.
Chemiluminescence methods are also used to measure the partial pressure of oxygen (DE 3702210). The method relies on the quenching effect of oxygen on the chemiluminescence of organic compounds that are embedded in a polymer matrix. In principle the method can be miniaturized and could be placed directly in the breathing gas. But there is conflicting information on the toxicity of the aromatic compounds that are used, since they might be introduced into the body during the measurement. In addition the method gives no linear relation between the oxygen partial pressure and the measuring signal. The signal course flattens already at medium partial pressure. Therefore the results get inaccurate at medium and especially at high partial pressures. In addition it is unknown jet, whether common anaesthetic agents like nitrous oxide and halogenated hydrocarbons influence the quenching effect, or cause a swelling of the matrix that leads to inaccurate signals.
It is accordingly a principal object of the present invention to provide an improved method and device to perform oxygen partial pressure measurements in breathing gas on a breath-by-breath basis, while avoiding the shortcomings and drawbacks of the prior art instruments and methods heretofore known.
It is the object of this invention to provide a simple, cost effective method and device that overcomes the problems referred to above. The method and the device allow the simple but accurate determination of different physiological parameters by the measurement of the partial pressure of oxygen in the mainstream of the breathing gas on a breath-by-breath basis.
According to the invented method to determine components of the breathing gas and thereby physiological parameters, an electrochemical sensor is placed in the mainstream of the breathing gas of a subject and measures the partial pressure of oxygen during one or several breathing cycles almost without time delay. The resulting electric sensor signal which is either directly proportional to the oxygen partial pressure or shows a mathematical relation to the oxygen partial pressure is transmitted to an electronic analyzer.
The course of the oxygen partial pressure can be determined for a part, a complete or several breathing cycles continuously. Upon the measured oxygen partial pressure values different physiologic parameters can be determined.
For instance the inspiratory and expiratory oxygen partial pressure can be determined from the measured oxygen partial pressure values.
The oxygen concentration of the breathing gas can be determined from the measured oxygen partial pressure and the known or measured pressure of the breathing gas.
Besides the expiratory flow can be measured simultaneously with a spirometer in the mainstream of the breathing gas. By integrating and multiplying both expiratory curves, the oxygen uptake of a subject can be calculated.
The metabolic rate and the oxygen consumption can be determined from the oxygen uptake per time.
Additional parameters can be determined from the oxygen consumption and the partial pressure of carbon dioxide or the carbon dioxide concentration, which can be determined in the expiratory breath by mainstream capnography. By applying an additional flow measurement of the breath, the amount of carbon dioxide produced by a subject in a certain time period can be calculated. For instance the respiratory quotient of a subject can be calculated from the oxygen consumption and the produced amount of carbon dioxide by dividing the exhaled carbon dioxide output with the oxygen uptake.
Moreover other gases, for instance oxygen, can be added to the breathing gas. If the subject only breathes oxygen for a certain time and then turns back to breath normal air, other physiologic parameters can be concluded from the signal time characteristics of the expiratory oxygen partial pressure.
The instant invention also allows for the determination of the subject""s heart frequency, by analyzing the oxygen partial pressure curve that has been recorded for one or several breathing cycles. The determined heart frequency and oxygen uptake also permit the measurement of the subject""s oxygen pulse.
The pattern of the oxygen partial pressure curve can reveal additional pathologic conditions of a subject. The amplitudes of the pulsatory changes in the oxygen partial pressure curve, that are caused by the heart beat during the exhalation, discloses further physiologic parameters.
It is another inventive step of this disclosure, that the breathing frequency can be derived from the time course of the oxygen partial pressure and other pathologic conditions from the breathing pattern. The analysis of the time course of the oxygen partial pressure for one or more breathing cycles allows for the determination of further pulmonary and/or cardiopulmonary parameters.
The device for the determination of physiologic parameters consists of an electrochemical oxygen sensor and an electronic analyzer. The sensor can be operated either in a galvanic or potentiostatic mode. The measurement is performed directly in the mainstream of the breathing gas. The sensors measures the oxygen partial pressure almost without time delay and transmits a signal, that is proportional to the oxygen partial pressure, to the electronic analyzer.
At a sudden change of the oxygen partial pressure the sensor shows a response time of less than 200 milliseconds until it reaches 90% of its end signal. The connected analyzer consists of a processor or a microcontroller and a graphic display.
The instant invention depicts a method and a device that neither show the considerable inaccuracy and time delay that is demonstrated by side stream methods, nor an interference from humidity or anaesthetic agents and the non linear response like optical systems. There is also no risk for the subject from dangerous substances, extreme operating conditions or the emission of electromagnetic radiation. In addition no interference with other measuring instruments can occur.